Signal transducer with distortion compensating amplifier



Jan. 6, 1953 N. a. sAUNDERs SIGNAL T RANSDUCER WITH DISTORTIONCOMPENSATION AMPLIFIER Filed June 25, 1946 Patented Jan. 6, 1.953

lUNITED STTES SIGNAL TRANSDUCER WITH DISTORTIN COMPENSATING AMPLIFIERApplication June 25, 1946, Serial No. 679,126

Claims. l

`This invention relates to an improved circuit for producing positivefeedback to reduce distortion of the wave form of an input signal. Inconnection with the use of certain types of radar equipment, it has beenfound that power amplifiers even of advanced design may introduce toomuch change in the wave form of the input signal.

An object of the invention is to devise improved means for providingpositive feedback with a View to more eifectively reducing distortion ofthe Wave form of an input signal which passes through a power amplifier.Other objects will appear from the following description.

It has been found by mathematical analysis that a certain definiteamount of positive feedback may reduce the fraction of distortionintroduced by an entire amplifier to that introduced by the circuitaround which the positive feedback may be placed. As illustrative of apreferred means of carrying this principle into effect, I haveillustrated the circuit arrangement shown in Figs. 1 and 2.

Figure 2 is a schematic circuit diagram showing the application of theinvention to a sweep circuit amplifier whose output supplies theexcitation for the azimuth synchro of a cathode ray position indicator.(A synchro is a device for the electrical transmission of angularposition; Technical Manual TM 11-467, War Department 1944, p. 314).structurally, a synchro has a slotted laminated iron stator having twosecondary windings resembling those on the stator of a two phaseinduction motor, and a single piimary Winding on the rotor. Thesecondary voltages vary with rotor position and may be used to excitethe deecting coils of a cathode r'ay -tube to indicate angular positionas shown on page 282 of the publication cited above. In such systemswhere high accuracy is desired, undesirable distortion s introduced bythe amplifier and by the iron losses in the synchro. It is an object ofthe invention to eliminate the distortion arising both in the amplifierand in the synchro and to supply a signal to the cathode ray positionindicator which is an exact reproduction of the input signal to theamplier.

In the preferred embodiment of the invention, the first two tubes of theamplier Tl and T2 are interconnected to provide a positive feedback fromthe output of T2 to the input of TI. This may be accomplished as isknown in the art by connecting the cathode 9 of tube T2 through asuitable resistor R1 to the cathode 8 of the input tube TI.- The amountof feedback may be ad- (Cl. Ri-380) justed by varying the respectivecathode resistors R2 and Rl. In the preferred adjustment, theregenerative or positive feedback is increased to the verge ofinstability; that is, the amplification of the first two tubes isadjusted for substantially infinite gain. In order to stabilize theamplifier as a whole when the first two stages are adjusted for infinitegain, it is necessary to provide a negative feedback from the amplieroutput stage to some point on the amplifier input. In the preferredembodiment this is accomplished by feedback circuit A connecting point20 in the output circuit of cathode fo-llower output stage Tt throughresistor Rli to point i@ in the cathode circuit of input stage Tl. Asecond negative feedback circuit B, to be described later, connectspoint It! through resistor R3 to synchro S2. Feedback resistors R3 andRd are in parallel With cathode resistor R2 between point It andreference potential point 2. The values of resistances R2, R3 and Re areselected to provide the desired value of negative feedback and at thesame time provide the required negative bias for tube TI. The value ofresistance R1 is selected to provide the desired value of positivefeedback.

Signal is introduced into the amplifier at the grid of the first tube.The amplified and inverted signal appears at the plate of this tubethrough the usual amplifying characteristics of a pentode having ananode and cathode resistor.

The signal from the plate of the first tube is passed directly to thegrid of the second tube T2 which is connected as a cathode follower tothe grid of the following power amplifier. A portion of this signalappears upon the cathode of the first tube producing regenerativefeedback as outlined above. The entire signal voltage from the cathodeof the second tube is passed through the coupling capacitor C3 to thegrid of the third tube T3 in conventional fashion.

The cathode of the third tube is connected to the reference potential 2through `an impedance R9 and C4 low at signal frequency and the plate ll! is connected to the supply potential 3U through a high impedance RI!!and RI i so that the signal appears amplified and inverted upon theplate it.

The coupling from the plate of the third tube to the grid of the fourthtube Tl! is again conventional. The fourth tube is a power amplifierconnected as a cathode follower with its plate connected directly to alow impedance potential supply. An impedance including cathode biasresistor Rit and primary windings 2l and 24 of synchros S2 and Sirespectively is connected between cathode IS and reference potential 2suchthat the proper bias is maintained upon the grid I6 at the normaloperating current while the impedance of the synchros with respect tothe reference potential at signal frequencies is high.

The signal introduced upon the grid of tube T4 appears substantiallyunaltered upon the cathode I9, but from an apparent source of low enoughimpedance to supply the current demanded by the load. A resistor R4returns from point 20 of the cathode circuit of tube T4 directly tocathode 8 of the rst tube so that a portion of the signal upon thecathode of the fourth tube, and hence of the output Signal, is returnedto the cathode of the rst tube. This signal is of the same polarity asthat originally applied to the grid of the rst tube so that itconstitutes negative feedback in path A as previously described.

The negative feedback stabilizes the entire amplifier even though thepositive feedback within the negative feedback makes the internal gainof the amplifier infinite. The critical amount of positive feedback isthat which makes the gain of the amplier infinite. This may be arrivedat mathematically by starting with the equation for the amplification ofan amplifier incorporating feedback:

This is a well-known equation and may be found in any standard text onRadio Engineering (see Termans Radio Engineering, page 248, secondedition, 1937). In the above equation Av represents the voltage gain ofan amplifier having feedback; A is the amplification of the amplifierwithout feedback; is the fraction of the output voltage which is fedinto the input. The sign of is positive for positive feedback, andnegative for negative feedback.

According ot the above equation it is necessary only to utilize a valueof which will cause A to be equal to 1 in which case the denominatorbecomes zero and the value of Av infinite. It will be apparent to oneskilled in the art that this condition can be brought about by feedingback just enough signal to the input to give the same output as thatwhich existed. More positive feedback is detrimental.

In Fig. 1 the distortion d of the various components and parts of thecircuit is given in percent, together with the feedback paths. The pathof principal interest according to this invention is negative feedbackpath B traced from point 2li of the amplifier output through primary andsecondary windings 2i and 22 or compensating synchro S2, resistor R3 topoint it of the amplifier input. This circuit feeds about 7% of theamplifier output voltage into the amplifier input in reverse phase. Thewave form applied to the input of a synchro is distorted owing to thehysteresis and eddy currents in the iron magnetic circuit of thesynchro. Synchros SI and S2 are identical and introduce identicalamounts of distortion. Since the distortion components introduced intothe amplifier input by S2 are in reverse phase, the action of thissynchro just cancels out the distortion introduced by output synchro Slbeyond the amplier output terminals. That is, synchro S2 compensates forthe distortion due to output synchro Si.

It is not possible to feedback directly from the output of the secondsynchro because both phase and amplitude of its output are constantlychanged mechanically.

It should be observed that the signal applied to the amplifier inputappears at its output as if supplied from a source of low internalimpedance. The negative feedback path B is the controlling path andinsures that the signal at the output is distorted in such a way that inpass-l ing through the output synchro it emerges an exact reproductionof the input signal to the amplifier. The output of the second synchrois thus undistorted as outlined above.

In an amplifier designed according to this invention as shown in Fig. 2,several additional feedback paths exist as indicated in paths D, E and Fof Fig. 1. Path D from the output cathode 9 of tube T2 provides apositive feedback of approximately 5% to grid il of T2. This path shownby the dotted arrows associated with tubes TI and T2 of Fig. 2, may betraced from cathode 9 of T2, R1, point Ill, condenser C2, grid 5 andgrid 6, plate 'l of Tl to grid Il of T2.

Path E provides a positive feedback of approximately 10% from point 20of amplifier output of grid Il of tube T4.

Path F provides positive feedback of approximately 87% via path E, justtraced, through grid il to point l on plate resistors RIU and Ril oftube T3, plate I4, condenser C5 to grid I6 of tube T4. The purpose ofpositive feedback, circuits E and F is to increase the amplification ofthe output stage T4.

It will be seen therefore that I have designed an eiicient positivemeans of providing feedback to reduce distortion. It should beunderstood that the invention is not limited to the specific formdisclosed. and may be modied in var1ous respects in accordance with thescope of the appended claims.

I claim:

1. in an amplier having a plurality of stages, an output stage of thecathode follower type, an input stage, and a regenerative feedbackconnection therebetween, said output stage including a tube having ananode, cathode and at least two grids and an output impedance connectedfrom said cathode to a source of negative potential, said input stageincluding a tube having an anode, cathode and at least one grid and ananode resistor connected from said anode to a source of positivepotential and having an intermediate potential point therein, aconnection from sa1d input tube anode to the rst grid of said outputtube, said regenerative connection including a connection from saidintermediate point to said second grid, and a condenser having lowimpedance at the operating frequency connected from said intermediatepotential point to the cathode end of said output impedance.

2. An amplier having a plurality of stages and including input andoutput circuits, an output device having a primary winding connected tothe output circuit and additional windings inductively coupled thereto,and means for compensating for the distortion arising within said outputdevice, said compensating means comprising a negative feedback circuitconnected from the junction of the output circuit and said output deviceto said amplifier input circuit and including therein a second devicehaving a primary winding connected to the output circuit and anadditional winding inductively coupled thereto, said primary winding,said additional Winding and their magnetic coupling circuit beingsubstantially identical with those of said output device, whereby thedistortion arising in said second device is similar to the distortion insaid output device and compensates therefor.

3. An amplifier having a plurality of stages and positive and negativefeedback circuits for eliminating distortion within the amplier, inputand output circuits and an output device having a primary windingconnected to the output circuit and additional windings inductivelycoupled thereto, and means for compensating for the distortion arisingwithin said output device, said compensating means comprising a negativefeedback circuit connected from the junction of the output circuit andsaid output device to said amplifier input circuit and including thereinan inductance coupling network having a magnetic circuit substantiallyequivalent to the magnetic circuit of said output device, whereby thereare produced substantially equivalent distortions in the feedbackcircuit as in the output device.

4. An amplifier having input and output circuits and positive andnegative feedback circuits for eliminating distortion within theamplifier and an output transducer including a first induotance couplingnetwork connected to the output circuit, and means for compensating forthe distortion arising within said output transducer, said compensatingmeans comprising a negative feedback circuit connected from the junctionof the output circuit and said output transducer to said amplifier inputcircuit and including therein a second inductance coupling networkhaving a magnetic circuit substantially equivalent to the magneticcircuit of said output transducer and rst inductance coupling network.

5. In an electrical system having an input circuit, an amplifier, and anoutput transducer connected to the output circuit of the amplifier, thecombination including positive and negative feedback means forcorrecting the distortion arising within the amplifier, and means forcompensating for the distortion generated in the transducer, saidcompensating means comprising a negative feedback circuit connected fromthe output circuit of the ampliiier to the input circuit thereof andincluding therein means for producing distortion substantially identicalwith said distortions generated within said transducer comprising adevice electrically equivalent to said output transducer.

NORMAICI B. SAUNDERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Radio Handbook, 6th edition,1939, pp. 318- 319.

Edward L. Ginzton: Balanced Feedback Amplifiers, Proceedings of theInstitute of Radio Engineers, Vol. 26, No. 11, November 1938, pp.1367-1379.

